Generally, chip resistors constituted of a chip-type insulating substrate and a resistant film provided on the upper surface thereof are not provided with sufficient surge resistance, and hence the resistance is prone to fluctuate when a surge voltage is applied, for example because of an influence of static electricity or a power source noise. For improving the surge resistance, extending a length of a path on the resistant film through which a current runs is known as an effective remedy.
Accordingly, a conventional chip resistor is provided with a terminal electrode on the respective longitudinal end portions on the upper surface of the substrate made of a heat-resistant insulating material such as a ceramic, and a resistant film located in a zigzag-folded shape between the terminal electrodes on the upper surface of the chip substrate for electrical connection, thus to secure a maximal length of the current path through the resistant film.
Under such structure, however, when a surge voltage is applied to the path between the terminal electrodes, discharge may take place between the zigzag-shaped resistant film and an inner edge of the terminal electrodes, by which the surge resistance of the resistant film is degraded.
A solution of this problem is provided by prior art disclosed in JP-A 2000-216001 and JP-A 2002-203702. Referring to FIGS. 8 and 9, a chip substrate 201 is provided with a terminal electrode 202, 203 located in a region close to respective edges 201a, 201b, and a resistant film 204 located between the terminal electrodes 202, 203, including a plurality of slits 211 that form the zigzag shape of the resistance film. In this chip resistor, the terminal electrodes 202, 203 respectively include a protrusion 205, 206 protruding from a portion of the inner edge 202a, 203a close to a side edge 201c of the chip substrate 201 toward the resistant film 204, and the resistant film 204 includes a lug 207, 208 formed at the respective end portions. The lugs 207, 208 are respectively disposed on or under the protrusions 205, 206 of the terminal electrodes 202, 203, so that the lugs 207, 208 overlap the protrusions 205, 206 for electrical connection, by which a gap 209, 210 is defined between the inner edge 202a, 203a of the terminal electrodes 202, 203 and the outer edge 204a, 204b of the resistant film 204. Such a structure prevents discharge between the inner edge 202a, 203a of the terminal electrode 202, 203 and the outer edge 204a, 204b of the resistant film 204, while securing a sufficient length of the current path through the resistant film 204.
Regarding a method of forming the zigzag-shaped resistant film, JP-A 2001-338801 proposes placing the resistant film of a certain width between the terminal electrodes such that the end portions of the resistant film in a longitudinal direction are electrically connected to the terminal electrodes respectively, by screen printing or the like. Simultaneously with the screen printing process, a first slit, which is a part of the foregoing plurality of slits, is formed on a side edge of the resistant film. Further, on the opposite side edge of the resistant film, a second slit is engraved through a processing work such as irradiation of a laser beam, subsequent to the formation of the resistant film. Such process can extend the current path in a zigzag pattern, through which the current runs from one of the terminal electrodes to the other.
In such process, the processing work such as the irradiation of a laser beam for engraving the second slit also includes a trimming adjustment for maintaining the resistance value of the resistant film within a predetermined tolerance, and is hence performed after the formation of the resistant film by screen printing or the like.
The prior art according to JP-A 2000-216001 or JP-A 2002-203702, however, has the following drawback arising from the structure that the side edges 205b, 206b of the protrusions 205, 206 of the terminal electrodes 202, 203, opposite to the outer side edges 205a, 206a close to the side edge 201c of the chip substrate 201, are orthogonal to the inner edges 202a, 203a of the terminal electrodes 202, 203.
When forming the resistant film 204 and the terminal electrodes 202, 203 disposed on the end portions of the latter by screen printing or the like, a positioning error is inevitably incurred therebetween, such as a case indicated by a double dashed chain line in FIG. 9, where the resistant film 204 is shifted with respect to the terminal electrodes 202, 203. Accordingly, the width W of the protrusions 205, 206 has to be sufficiently large, so as to keep the lugs 207, 208 of the resistant film 204 from passing over the inwardly facing side edges 205b, 206b of the protrusions 205, 206, even with an assumed maximum positioning error.
Whereas, making the width W larger, with the respective inwardly facing side edges 205b, 206b of the protrusions 205, 206 of the terminal electrodes 202, 203 oriented orthogonal to the inner edges 202a, 203a of the terminal electrodes 202, 203, reduces a length L′ of a portion of the outer edges 204a, 204b of the resistant film 204 opposing the inner edge 202a, 203a of the terminal electrodes 202, 203, in other words the length of the gaps 209, 210 serving for preventing the discharge is reduced by the same amount that the width W of the protrusions 205, 206 is increased. Consequently, the length of the current path of the resistant film 204 is reduced, and the surge resistance of the resistant film 204 is thereby degraded.
In addition, referring to the formation of the second slit on the resistant film by the processing work according to the prior art proposed in JP-A2001-338801, when the position to engrave the second slit is shifted in a widthwise direction of the slit, the width between the second slit and the first slit simultaneously formed with the resistant film, and also the gap between the second slit and the terminal electrodes fluctuate to a wider or narrower side. This results in fluctuation in resistance value of the resistant film.
A conventional solution of the above problem is shooting an entirety of the chip substrate by a camera, and determining a position to engrave the second slit on the image, based on the overall shape of the resistor. However, since a positional shift incurred in the screen printing process of the resistant film may be added to the positioning error for the second slit in a widthwise direction of the slit, the total positioning error may become excessively large, thus to exceed the tolerance in positioning error. Consequently, the rate of defective products having a resistance value deviated from a predetermined range becomes higher.
Besides, it takes considerable time in determining the position to be engraved in the image of the entire resistant film, before performing the processing work of engraving the second slit, which naturally incurs an increase in cost.